INTRODUCTION. 



Xli 



The course of development is subject to one or two 

 variations. In some cases the entire body of the embryo 

 on becoming attached spreads out into a circular disk, from 

 the centre of which the stem is subsequently developed. 

 In Tubularia (Plate XX. figs, c, d) and in Coryne Van-Benc- 

 denii (p. 46) the planule stage is wanting, and the embryo 

 takes on the form of the polypite before leaving the gono- 

 phore. When it escapes from its confinement the body is 

 furnished with an oral aperture at one end, surrounded by 

 a number of tentacles ; it continues locomotive for a short 

 time, and then fixes itself by the aboral extremity, and 

 developes a stem and the full complement of arms. 



In some of the species which are furnished with a 



Fis. xvii. 



d 



The gonotlieca of Sertularia cupressina. , b, c. The capsule crowned by 

 the marsupium in various stages of development, d. The capsule with the 

 marsupium ruptured for the escape of the planulse. 



chitinous receptacle for the protection of the gonophores 

 (Thecaphora] , the ova at a certain stage are transferred to 

 a kind of nest or marsupial sac enveloped in a thick 



